Frank shared a ringoscillator made from a CPLD. It creates a ~18MHz clock with no crystal or external oscillator:
A ringoscillator can be implemented through a shiftregister with an inverted input.
The ringoscillator is feeding a counter and the counter is feeding the two leds.
The most critical part was to prevent xst from optimizing away the ringoscillator. This is achieved by the “keep” syntheses attributes.